Fuel cells and redox flow batteries (RFB) are powerful energy storage technologies that rely on selective transport of ions across a membrane in order to generate electrical current. Presently, the standard materials used for membranes in these technologies are perfluorinated sulfonic acid (PFSA) materials, such those as marketed under the name NAFION. These PFSA materials are desirable for their resistance to harsh environments and ion-conducting properties. However, these materials are relatively expensive and further improvements in operating properties are desirable.
Silicate proton-conducting materials are known but are not sufficiently developed so as to be commercially viable.
One common mechanism for proton transport is Grottuss hopping, wherein hydronium ions “hop” from water molecule to water molecule. This mechanism requires that water be sufficiently free to rotate and diffuse in order to promote solvation of a hydronium ion. Therefore, the design of a silicate proton-conducting material should have pores of sufficient size to promote the molecular motion of water (i.e., facilitate hydronium solvation and proton hopping). This size should be larger than a single water molecule (i.e., radius of 0.138 nm) but not so large as to facilitate diffusive transport of other molecules.
Accordingly, the development of robust, inexpensive proton-conducting materials is desired, yet significant compositional and structural limitations presently prevent the use of silicate materials as replacements for PFSA materials.